5000 rpm, Falls on its face

[dmaddox]

Just when I thought I had the 2.0 dialed in - Less than 200 miles later, its now falling on its face at 5000rpm. (1968 911L).

So, I checked the distributor cap, rotor (both new) they look fine. I cleaned them nonetheless...I then replaced both fuel filters with oem Bosch filters. I then checked fuel was being supplied properly - it is. So, it must be spark, no?

A total power cut off at 5000 rpm. Almost identical to a rev limiter. I don't stall, sputter or cough. It behaves exactly like a rev limiter.

Could something have gone wrong in the distributor? (Rebuilt), perhaps an ignition issue? I have changed nothing, this behavior just began.

Any tips are appreciated.

Thank you.

[Richy]

Do you have Pertronix ignition or points ? New points have a spring that is too weak and acts like a rev limiter , you need old or NOS . I've never had luck with Pertronix without a CD in SWB cars . Harmonics or something in the 1 switched circuit to the engine compartment . Do you have a coil made for the ballast resistor ?

[frederik]

Do you have a rev-limiting rotor? It might be cutting off too soon. If this happens you get a loud bang in the exhaust due to the unburned fuel.

[dmaddox]

Regular rotor, and no loud bang. I have a new pertronix with a new permatune coil and 2nd gen permatune. Was running fine until this weekend, it started capping at 5k rpm.

[Xavier PJB]

Do not know for yours, but my 2.0S had similar issue last year, and that was coming from the points. I replaced them and...problem disappeared, and reappeared again 3000 miles after, and points again...
Found out that the MSD (6AL) with the high voltage coil are "eating" fast the points. not sure it helps, but sharing experience

[dmaddox]

Inspected the Pertronix, rotor - again, everything looks new. Any pertronix I have had go bad in the past is obvious when looking at it (shorts, burns, etc.).

The only thing in the circle I can point my finger at is the Permatune - or perhaps the coil. I am not sure how to test either of the two without finding / buying more stuff and replacing it. If the MSD modules were not the size of Texas, I'd consider one of those. Buying a used Permatune - unless from a trusted member is sketchy in my opinion.

[1QuickS]

I just removed two running Permatunes and Bosch coils from my car if interested.

Before all of that check ignition timing. Possibly the dizzy rotated and timing changed.

[dmaddox]

That is fantastic that you have these available! I will double check timing and report back soon. Thank you Paul.

[dmaddox]

I finally had an opportunity to warm up the car and get some readings. Timing is spot on. It is about 5 degrees advanced at idle and right between the lines at 6000 rpm. To which I asked......how was I able to rev it to 6000 rpm? So while idling not under load, it idles, revs and behaves perfectly. I don't mean to brag - but with the help of this forum (Paul, Ed, others) I have this 2.0 seriously dialed in. It runs like a top. However, under load, she's falling dead at 5000 rpm. It doesn't stall...it doesn't cough. Just flat lines, again...like a rev limiter. I have checked the rotor (I have two, so I put a drop of super glue on one so the spring loaded rev limiter wouldn't move), and the behavior is the same.

So, perhaps either fuel pump (since the filters are new, even noting the old ones didn't have any sediment/goo), clog some where (or possibly a hidden filter up by the tank?).....or ignition module/coil would be my next guess. Today's test rules out pertronix and distributor.

So odd, its perfect at idle and revving when sitting. Sounds perfect, too.

@1QuickS I may take you up on your offer, if you are willing to sell, just to rule out Permatune and/or coil.

Edit: Thinking more about this - I am wondering how load/running on the street....how would that affect spark? The fact that I can rev it cleanly while sitting - doesn't that rule out all of the ignition components, leaving nothing more at this point than fuel delivery, which would make sense?

[1QuickS]

I've had a surprising amount of recent discussion with users suffering deficient fuel delivery issues.

I recommend checking fuel flow using the following setup to replicate real time fuel delivery:

• disconnect fuel line to one of the banjo fittings at the carbs
• using 5/15" "tee" fitting install a fuel pressure gauge into the line
• on the third connection of the "tee" run a line into a fuel catch tank
• place a pair of vise grip pliers (needle nose if you have them) onto the line to the catch tank
• turn on ignition and adjust vise grip pliers until 3.5 psi is indicated on the fuel pressure gauge
• fuel flow in gallons/hour (0.036 oz/second) should be equal to 1/10th of rated engine HP (a 160 HP engine would need 16 GPH fuel flow)